1. Field of the Invention
The present invention relates to polarization independent grating modulator. More particularly, the present invention relates to micromachined grating modulators which exhibit polarization independent behavior.
2. Description of Related Art
Optical modulators are an important component in optical systems for controlling and modulating light. In particular, for fiber-optic networks, modulators are used for imparting data modulation on the transmitting laser beam and as an electronically controlled variable optical attenuator (VOA) for channel equalization and power control. In fiber-optic networks the state of polarization is unknown and therefore little or no polarization dependence is tolerated from components.
Bloom et al. (U.S. Pat. No. 5,311,360) demonstrated a micromachined grating modulator comprised of narrow ribbons anchored at the two ends but suspended in the center xcex/2 (half wavelength) above the substrate. The ribbons are separated by gaps of the same width. Both ribbon and gap have a reflective coating from which light is reflected in phase and therefore it emulates a mirror. By applying a voltage to the ribbons, the electrostatic force moves the ribbon down by xcex/4. Now the ribbon and gap are out of phase and all the light is diffracted out in multiple orders. Thus modulation is achieved.
One limitation of the previous invention is that the height difference between the ribbon and gap leads to poor spectral performance. Bloom et al. (U.S. Pat. No. 5,841,579) improved on this by inventing a flat grating light valve comprised of ribbons of equal width with very little gap between them. In the nominal position, all ribbons are at the same height. By applying a voltage and pulling every other ribbon down, the grating is turned on.
For fiber-optic applications operating over the bandwidth of erbium doped fiber amplifier (EDFA), the spectral performance of the previous invention is not acceptable especially at high attenuation. Godil et al. (Achromatic optical modulator, U.S. patent application Ser. No. 09/372,649, filed Aug. 11, 1999, now U.S. Pat. No. 6,169,624) demonstrated a device with alternate narrow and wide ribbon. By proper choice of the ribbon widths and gap width, spectrally flat attenuation over the EDFA band over a large dynamic range is obtained.
A limitation of the previous inventions, because of lack of symmetry, is that they are not completely polarization independent. In particular, at high attenuation the polarization dependence is unacceptably high for fiber-optic networks.
What is needed is a micromachined modulator which exhibits achromatic and polarization independent behavior.
The present invention is directed towards a fiber-optic modulator comprising of an input optical fiber carrying a light beam through a lens onto a micromachined reflective modulator, back through the lens into an output optical fiber. The micromachined modulator is a two dimensional grating or periodic structure which is modulated by the application of a voltage. The two dimensional grating is symmetric in the X and Y axes, and therefore leads to polarization independent behavior. The achromatic modulator invention of Godil (patent application filed August 1999) is also incorporated to give achromatic behavior.